(1) State of the Invention
The present invention relates to compositions containing a fluorescent compound and a stable 1,2-dioxetane which can be triggered by chemical reagents including enzymes and generate enhanced chemiluminescence. In particular the present invention relates to a method for significantly enhancing the chemiluminescence which involves intermolecular energy transfer to a fluorescent compound in an organized assembly, such as a micelle, which maintains a close spacing between the dioxetane and the fluorescent compound.
(2) Prior Art
1. Mechanisms of Luminescence.
Exothermic chemical reactions release energy during the course of the reaction. In virtually all cases, this energy is in the form of vibrational excitation or heat. However, a few chemical processes generate light or chemiluminescence instead of heat. The mechanism for light production involves thermal or catalyzed decomposition of a high energy material (frequently an organic peroxide such as a 1,2-dioxetane) to produce the reaction product in a triplet: or singlet electronic excited states. Fluorescence of the singlet species results in what has been termed direct chemiluminescence. The chemiluminescence quantum yield is the product of the quantum yields for singlet chemiexcitation and fluorescence. These quantities are often expressed as efficiencies where efficient (%)=.phi..times.100. Energy transfer from the triplet or singlet product to a fluorescent acceptor can be utilized to give indirect chemiluminescence. The quantum yield for indirect chemiluminescence is the product of the quantum yields for singlet or triplet chemiexcitation, energy transfer, and fluorescence of the energy acceptor. ##STR1##
2. Dioxetane Intermediates in Bioluminescence.
In 1968 McCapra proposed that 1,2-dioxetanes might be the key high-energy intermediates in various bioluminescent reactions including the firefly system. (F. McCapra, Chem. Commun., 155 (1968)). Although this species is apparently quite unstable and has not been isolated or observed spectroscopically, unambiguous evidence for its intermediacy in the reaction has been provided by oxygen-18 labeling experiments. (O. Shimomura and F. H. Johnson, Photochem. Photobiol., 30, 89 (1979)). ##STR2##
3. First Synthesis of Authentic 1,2-Dioxetanes.
In 1969 Kopecky and Mumford reported the first synthesis of dioxetane (3,3,4-trimethyl-1,2-dioxetane) by the base-catalyzed cyclization of a beta-bromohydroperoxide. (K. R. Kopecky and C. Mumford, Can. J. Chem., 47, 709 (1969)). As predicted by McCapra, this dioxetane did, in fact, produce chemiluminescence upon heating to 50.degree. C. with decomposition to acetone and acetaldehyde. However, this peroxide is relatively unstable and cannot be stored at room temperature (25.degree. C.) without rapid decomposition. In addition, the chemiluminescence efficiency is very low (less than 0.1%). ##STR3##
Bartlett and Schaap and Mazur and Foote independently developed an alternative and more convenient synthetic route to 1,2-dioxetanes. Photooxygenation of properly-substituted alkenes in the presence of molecular oxygen and a photosensitizing dye produces dioxetanes in high yields. (P. D. Bartlett and A. P. Schaap, J. Amer. Chem. Soc., 92, 3223 (1970) and S. Mazur and C. S. Foote, J. Amer. Chem. Soc., 92, 3225 (1970)). The mechanism of this reaction involves the photochemical generation of a metastable species known as singlet oxygen which undergoes 2+2 cycloaddition with the alkene to yield the dioxetane. Research has shown that a variety of dioxetanes can be prepared using this reation (A. P. Schaap, P. A. Burns, and K. A. Zaklika, J. Amer. Chem. Soc., 99, 1270 (1977); K. A. Zaklika, P. A. Burns, and A. P. Schaap, J. Amer. Chem. Soc., 100, 318 (1978); K. A. Zaklika, A. L. Thayer, and A. P. Schaap, J. Amer. Chem. Soc., 100, 4916 (1978); K. A. Zaklika, T. Kissel, A. L. Thayer, P. A. Burns, and A. P. Schaap, Photochem. Photobiol., 30, 35 (1979); and A. P. Schaap, A. L. Thayer, and K. Kees, Organic Photochemical Synthesis, II, 49 (1976)). During the course of this research, a polymer-bound sensitizer for photooxygenations was developed (A. P. Schaap, A. L. Thayer, E. C. Blossey, and D. C. Neckers, J. Amer. Chem. Soc., 97, 3741 (1975); and A. P. Schaap, A. L. Thayer, K. A. Zaklika, and P. C. Valenti, J. Amer. Chem. Soc., 101, 4016 (1979)). This new type of sensitizer has been patented and sold under the tradename SENSITOX.TM. (U.S. Pat. No. 4,315,998 (Feb. 16, 1982); Canadian Patent No. 1,044,639 (Dec. 19, 1979)). Over fifty references have appeared in the literature reporting the use of this product. ##STR4##
4. Preparation of Stable Dioxetanes Derived from Sterically Hindered Alkenes.
Wynberg discovered that photooxygenation of sterically hindered alkenes such as adamantylideneadamantane affords a very stable dioxetane (J. H. Wieringa, J. Strating, H. Wynberg, and W. Adam, Tetrahedron Lett., 169 (1972)). A collaborative study by Turro and Schaap showed that this dioxetane exhibits an activation energy for decomposition of 37 kcal/mol and a half-life at room temperature (25.degree. C.) of over 20 years (N. J. Turro, G. Schuster, H. C. Steinmetzer, G. R. Faler, and A. P. Schaap, J. Amer. Chem. Soc., 97, 7110 (1975)). In fact, this is the most stable dioxetane yet reported in the literature. Adam and Wynberg have recently suggested that functionalized adamantylideneadamantane 1,2-dioxetanes may be useful for biomedical applications (W. Adam, C. Babatsikos, and G. Cilento, Z. Naturforsch., 39b, 679 (1984); H. Wynberg, E. W. Meijer, and J. C. Hummelen, In Bioluminescence and Chemiluminescence, M. A. DeLuca and W. D. McElroy (Eds.) Academic Press, New York, p. 687, 1981; and J. C. Hummelen, T. M. Luider, and H. Wynberg, Methods in Enzymology, 133B, 531 (1986)). However, use of this extraordinarily stable peroxide for chemiluminescent labels requires detection temperatures of 150 to 250.degree. C. Clearly, these conditions are unsuitable for the evaluation of biological analytes in aqueous media. McCapra, Adam, and Foote have shown that incorporation of a spirofused cyclic or polycyclic alkyl group with a dioxetane can help to stabilize dioxetanes that are relatively unstable in the absence of this sterically bulky group (F. McCapra, I. Beheshti, A. Burford, R. A. Hann, and K. A. Zaklika, J. Chem. Soc., Chem. Commun., 944 (1977); W. Adam, L. A. A. Encarnacion, and K. Zinner, Chem. Ber., 116, 839 (1983); G. G. Geller, C. S. Foote, and D. B. Pechman, Tetrahedron Lett., 673 (1983); P. Lechtken, Chem. Ber., 109, 2862 (1976); and P. D. Bartett and M. S. Ho, J. Amer. Chem. Soc., 96, 627 (1974)) ##STR5##
5. Effects of Substituents on Dioxetane Chemiluminescence.
The stability and the chemiluminescence efficiency of dioxetanes can be altered by the attachment of specific substituents to the peroxide ring (K. A. Zaklika, T. Kissel, A. L. Thayer, P. A. Burns, and A. P. Schaap, Photochem. Photobiol., 30, 35 (1979); A. P. Schaap and S. Gagnon, J. Amer. Chem. Soc., 104, 3504 (1982); A. P. Schaap, S. Gagnon, and K. A. Zaklika, Tetrahedron Lett., 2943 (1982); and R. S. Handley, A. J. Stern, and A. P. Schaap, Tetrahedron Lett., 3183 (1985)). The results with the bicyclic system shown below illustrate the profound effect of various functional groups on the properties of dioxetanes. The hydroxy-substituted dioxetane (X.dbd.OH) derived from the 2,3-diaryl-1,4-dioxene exhibits a half-life for decomposition at room temperature (25.degree. C.) of 57 hours and produces very low levels of luminescence upon heating at elevated temperatures. In contrast, however, reaction of this dioxetane with a base at -30.degree. C. affords a flash of blue light. Kinetic studies have shown that the deprotonated dioxetane (X.dbd.O.sup.-) decomposes 5.7.times.10.sup.6 times faster than the protonated form (X.dbd.OH) at 25.degree. C. ##STR6##
The differences in the properties of these two dioxetanes arise because of two competing mechanisms for decomposition (K. A. Zaklika, T. Kissel, A. L. Thayer, P. A. Burns, and A. P. Schaap, Photochem. Photobiol., 30, 35 (1979); A. P. Schaap and S. Gagnon, J. Amer. Chem. Soc., 104, 3504 (1982); A. P. Schaap, S. Gagnon, and K. A. Zaklika, Tetrahedron Lett., 2943 (1982); and R. S. Handley, A. J. Stern and A. P. Schaap, Tetrahedron Lett., 3183 (1985)). Most dioxetanes cleave by a process that involves homolysis of the O--O bond and formation of a biradical. An alternative mechanism is available to dioxetanes bearing substituents such as O.sup.- with low oxidation potentials. The cleavage is initiated by intramolecular electron transfer from the substituent to the antibonding orbital of the peroxide bond.
6. Chemical Triggering of Dioxetanes.
The first example in the literature is described above (A. P. Schaap and S. Gagnon, J. Amer. Chem. Soc., 104, 3504 (1982)). However, the hydroxy-substituted dioxetane and any other examples of the dioxetanes derived from the diaryl-1,4-dioxenes are far too unstable to be of use in any application. They have half-lives at 25.degree. C. of only a few hours. Neither the dioxetane nor the precursor alkene would survive the conditions necessary to prepare derivatives. Further, these non-stabilized dioxetanes are destroyed by small quantities of amines (T. Wilson, Int. Rev. Sci.: Chem., Ser. Two, 9, 265 (1976)) and metal ions (T. Wilson, M. E. Landis, A. L. Baumstark, and P. D. Bartlett, J. Amer. Chem. Soc., 95, 4765 (1973); P. D. Bartlett, A. L. Baumstark, and M. E. Landis, J. Amer. Chem. Soc., 96, 5557 (1974) and could not be used in the aqueous buffers required for enzymatic triggering.
7. Energy-Transfer Chemiluminescence Involving Dioxetanes in Homogeneous Solution.
The first example of energy-transfer chemiluminescence involving dioxetanes was described by Wilson and Schaap (T. Wilson and A. P. Schaap, J. Amer. Chem. Soc., 93, 4126 (1971)). Thermal decomposition of a very unstable dioxetane (cis-diethoxydioxetane) gave both singlet and triplet excited ethyl formate. Addition of 9,10-diphenylanthracene and 9,10-dibromoanthracene resulted in enhanced chemiluminescence through singlet-singlet and triplet-singlet energy-transfer processes, respectively. These techniques have subsequently been used by many other investigators to determine yields of chemiexcited products generated by the thermolysis of various dioxetanes (For a review, see W. Adam, In Chemical and Biological Generation of Excited States, W. Adam and G. Cilento, Eds. Ch. 4, Academic Press, New York, 1982). Energy transfer in homogeneous solution, however, requires high concentrations of the energy acceptor because of the short lifetimes of the electronically excited species. These high concentrations lead to problems of self-quenching and reabsorption. The present invention solves the problem by using the 1,2-dioxetane and a fluorescent energy acceptor which are preferably both incorporated in a micelle affording efficient energy transfer without the need for high concentrations of a fluorescer in bulk solution.
8. Enhanced Chemiluminescence from a Dioxetane Using Intermolecular Energy in Micelles.
Rates of various chemical reactions can be accelerated by micelles in aqueous solution (See, for example: E. H. Cordes and R. B. Dunlap, Acc. Chem. Res., 2,329 (1969)). Catalysis results from solubilization of the substrate in the micellar pseudophase and from electrostatic, hydrophobic, or polarity factors. Aqueous micelles have been used to increase the rate of chemically triggered dioxetanes (A. P. Schaap, Final Technical Report to the Office of Naval Research, 1987, page 16). No experiments using fluorescent compounds such as co-surfactants to enhance chemiluminescence efficiency are reported.
Several reports describe enhanced chemiluminescence from chemical reactions in micellar environments. However, none of these make use of energy transfer to a fluorescent co-surfactant. No stabilized dioxetanes have been studied in micelles. Goto has investigated the chemical oxidation of a luciferin in the presence of neutral, anionic, and cationic surfactants (T. Goto and H. Fukatsu, Tetrahedron Lett., 4299 (1969)). The enhanced chemiluminescence was attributed to an increase in the fluorescence efficiency of the reaction product in the micelle compared to aqueous solution. The effect of cetyltrimethylammonium bromide micelles on the chemiluminescent reaction of acridan esters in aqueous alkaline solution has been reported (F. McCapra, Acc. Chem. Res., 9, 201 (1976). McCapra indicates, however, that micellar environment does not "assist the excitation reaction". Rather, the micelles are thought to enhance the luminescent yield by decreasing the rate of a competing, non-luminescent hydrolytic reaction. Similarily, Nikokavouras and Gundermann have studied the effect of micelles on chemiluminescent reactions of lucigenin and luminol derivatives, respectively (C. M. Paleos,G. Vassilopoulos, and J. Nikokavouras, Bioluminescence and Chemiluminescence, Academic Press, New York, 1981, p. 729; K. D. Gundermann, Ibid., p. 17). Shinkai observed that chemiluminescence yields from unstable, non-isolable dioxetanes could be enhanced in micelles relative to water (S. Shinkai, Y. Ishikawa, O. Manabe, and T. Kunitake, Chem. Lett., 1523 (1981). These authors suggested that the yield of excited states may be higher in the hydrophobic core of micelles than in water.
The only reference to enhancement of enzymatically generated chemiluminescence with surfactants involves the work of Kricka and DeLuca on the firefly luciferase system (L. J. Kricka and M. DeLuca, Arch. Biochem. Biophys., 217, 674 (1983)). Nonionic detergents and polymers enhanced the total light yield by increasing the turnover of the enzyme. Cationic surfactants such as (cetyltrimethylammonium bromide, CTAB) actually resulted in complete inhibition of the catalytic activity of the luciferase.
A method for enhancing the chemiluminescent yield of the luminol/peroxidase reaction by addition of 6-hydroxybenzothiazole derivatives or para-substituted phenols (G. H. G. Thorpe, L. J. Kricka, S. B. Moseley, T. P. Whitehead, Clin. Chem., 31, 1335 (1985); G. H. G. Thorpe and L. J. Kricka, Methods in Enzymology, 133, 331 (1986); and L. J. Kricka, G. H. G. Thorpe, and R. A. W. Stott, Pure & Appl. Chem., 59, 651 (1987)). The mechanism for the enhancement is not known but it does not involve intramolecular energy transfer or intermolecular transfer to a co-micellar fluorescent surfactant.
Co-micellar fluorescent probes have been used to study the dynamic properties of micelles (Y. Kubota, M. Kodama, and M. Miura, Bull. Chem. Soc.. Jpn., 46, 100 (1973); N. E. Schore and N. J. Turro, J. Amer. Chem. Soc., 96, 306 (1974); and G. W. Pohl, Z. Naturforsch., 31c, 575 (1976)). However, no examples appear in the literature of using these fluorescent materials to enhance chemiluminescent reactions in micelles through energy-transfer processes.
9. Chemiluminescent Immunoassays.
There are no reports of dioxetanes as enzyme substrates or their use in enzyme-linked assays prior to the filing date of Ser. No. 887,139. Wynberg has used stable dioxetanes as "thermochemiluminescent" labels for immunoassays (J. C. Hummelen, T. M. Luider, and H. Wynberg, Methods in Enzymology, 133B, 531 (1986)). These dioxetanes are used to label biological materials such as proteins. Assays are subsequently conducted by heating the sample at 100 to 250.degree. C. and detecting the thermally generated chemiluminescence. This technique is distinctly different from the use of triggerable dioxetanes as enzyme substrates.
Luminol derivatives, acridinium esters and lucigenin have been employed as chemiluminescent labels for antigens, antibodies, and haptens (H. R. Schroeder and F. M. Yeager, Anal. Chem., 50, 1114 (1978); H. Arakawa, M. Maeda, and A. Tsuju, Anal. Biochem., 79, 248 (1979); and H. Arakawa, M. Maeda, and A. Tsuji, Clin. Chem., 31, 430 (1985). For reviews, see: L. J. Kricka and T. J. N. Carter, In Clinical and Biochemical Luminescence, L. J. Kricka and T. J. N. Carter (Eds.), Marcel Dekker, Inc., New York, 1982, Ch. 8; L. J. Kricka, Ligand-Binder Assays, Marcel Dekker, Inc., New York, 1985, Ch. 7; F. McCapra and I. Beheshti, In Bioluminescence and Chemiluminescence: Instruments and Applications, Vol. I, K. Van Dyke (Ed.), CRC Press, Inc., Boca Raton, Fla., 1985, Ch. 2, Note, in particular, the section on dioxetanes, p. 13; and G. J. R. Barnard, J. B. Kim, J. L. Williams, and W. P. Collins, Ibid, Ch. 7). Assay systems involving the use of enzyme-labeled antigens, antibodies, and haptens have been termed enzyme immunoassays. The enzyme labels have been detected by color or fluorescence development techniques. More recently, luminescent enzyme immunoassays have been based on peroxidase conjugates assayed with luminol/hydrogen peroxide, pyrogallol/hydrogen peroxide, Pholas dactylus luciferin, or luminol under alkaline conditions (L. J. Kricka and T. J. N. Carter, In Clinical and Biochemical Luminescence, L. J. Kricka and T. J. N. Carter (Eds.), Marcel Dekker, Inc., New York, 1982, Ch. 8). No enzyme-linked assays have described dioxetanes as enzymatic substrates to generate light for detection prior to my application Ser. No. 887,139.
10. Photographic Detection of Luminescent Reactions.
Instant photographic film and x-ray film have been used to record light emission from several chemiluminescent and bioluminescent reactions (L. J. Kricka and G. H. G. Thorpe, Methods in Enzymology, 133, 404 (1986) and references therein. See also: M. M. L. Leong, C. Milstein, and R. Pannel, J. Histochem. Cytochem., 34, 1645 (1986); R. A. Bruce, G. H. G. Thorpe, J. E. C. Gibbons, P. R. Killeen, G. Ogden, L. J. Kricka, and T. P. Whitehead, Analyst, 110, 657 (1985); J. A. Matthews, A. Batki, C. Hynds, and L. J. Kricka, Anal. Biochem., 151, 205 (1985); and G. H. G. Thorpe, T. P. Whitehead, R. Penn, and L. J. Kricka, Clin. Chem., 30, 806 (1984). No examples appear in the literature on the photographic detection of chemiluminescence derived from chemical or enzymatic triggering of stabilized dioxetanes prior to my application Ser. No. 887,139.